EXPERIMENTAL SPEED OF SOUND MEASUREMENTS IN LIQUID METHANE AT CRYOGENIC TEMPERATURE

The density of liquefied natural gas (LNG) is usually being calculated based on the composition and an equation of state. In the absence of an accurate composition measurement, the density is also commonly, but roughly, determined by the Coriolis or Ultrasonic Flow Meter (USM) used for the flow rate measurements. The calculation of density has a rather large uncertainty (between 0.2% - 0.5%, not including the uncertainty of the composition measurement), be- cause the applied Equations of State are not yet validated at a sufficient low level of uncertainty. As a part of an on-going European research project (called ”Metrology for LNG III - Metro- logical support for LNG and LBG as a transport fuel”) to promote the large scale use of LNG and liquefied biogas (LBG) as a transport fuel, the Italian Metrological Institute (INRiM) has developed a new on-line density and speed of sound ultrasonic sensor. The novel sensors, having the aim to improve the accuracy and reduce the measurement uncertainty of LNG density and speed of sound measurements, can work both in laboratory controlled conditions and in-field, it will be used for disseminating traceable density measurement when it is mounted on the same industrial circuits where commercial sensors are installed. Calibration tests have been carried out both at ambient temperature, using water, and at cryogenic temperature in a closed-loop cryostat, using methane. In this paper accurate speed of sound measurements in liquid-phase methane in the tempera- ture range of (100 and 150) K and for pressures up to 10 MPa are reported. These results, with an associated uncertainty in the order of 0.1%, were compared with literature values and with predictions of the dedicated equation of state.